Heterogeneous integration involves the assembly of separately manufactured components to provide enhanced functionality. This strategy requires the combination of a broad variety of different materials with different characteristics. Redistribution layers (RDLs) consisting of Cu conductor lines and organic dielectric are key components of a variety of packaging technologies within this approach to connect the various functional components. In particular, fan out wafer level packaging (FOWLP) was developed as an enhancement to standard WLP to allow for higher performance, decreasing form factor, and significant cost reduction. Further decreasing lines and spaces as well as multilayer RDLs allow for increasing the I/O count and, therefore, for more complex, high speed dies to be assembled within a heterogeneous package. Next generation devices with FOWLP require decreasing the RDL pitch down to 1 µm x 1 μm or even below, which involves challenges in terms of reliability of the Cu conductor lines. With decreasing diameter and pitch of RDL Cu lines, mechanical properties become increasingly important. This is especially true with regard to the variety of different materials, which are in direct contact upon heterogeneous integration of different components. In order to improve the reliability of such assembly, the mechanical properties of the individual materials need to be optimized. However, this improvement is limited by issues upon scaling down the Cu conductor lines. Thus, a different approach is required to produce packages of sufficient reliability. In addition to optimization of the individual materials, a holistic approach would include the surrounding materials within the package. Recent findings emphasize the importance of the formation of a proper composite with the adjacent dielectric. Thereby, the mechanical properties and, thus, the reliability of the Cu conductor lines, may be strongly improved. Utilization of the surrounding materials of the heterogeneous package by proper composite formation constitutes a promising strategy to fulfill reliability criteria of upcoming FOWLP applications. Properly prepared composites may benefit from synergistic effects and overcome the limitations of the individual components. Different chemistry solutions will be discussed with respect to their impact on composite formation as well as suitability for potential 5G applications.